1. Field of Invention
This invention pertains generally to inertial measurement, and, more particularly, to an inertial measurement system and method in which the fixed bias offset of rate sensors or gyroscopes is cancelled.
2. Related Art
Angular rate sensors or gyroscopes (gyros) exhibit a non-zero output in the absence of rotation about the input axis. This non-zero output is called bias, and it can cause significant errors in inertial measurements. The magnitude of the error depends upon the type of sensor or gyroscope used. In applications with stringent limits on offset errors, more expensive sensors with very low residual offset errors must generally be used, and those sensors are often relatively large and heavy.
Heretofore, there have been some attempts to eliminate offset errors by rotating a gyro about the input axis of the sensor, which is bad because it produces a rotation rate signal that must be dealt with. In other systems, horizontal inertial instruments such as gyroscopes and accelerometers have been rotated continuously in carousel fashion about a vertical axis. However, it is not always feasible or even desirable to apply a constant rotation to such instruments.
With ring laser gyros, certain errors have been eliminated by dithering the sensors about the input axis. However, as noted above, rotating a sensor about its input axis is undesirable because it provides an angular rate input other than the one to be detected.
In inertial systems of the prior art it is also necessary to rely on complex models to predict variation in offsets over time and due to factors such as temperature. These models can be quite useful in some applications, but it is often necessary to cancel offsets to a higher level of precision than the models can provide.